In modern communication methods, the data to be transmitted are both phase modulated and amplitude modulated onto a carrier signal. Very often, digital types of modulation are used for such communication methods. Examples of these are quadrature amplitude modulation (QAM), “Quadrature Phase Shift Keying” (QPSK) or Orthogonal Frequency Division Multiplexing (OFDM). To be able to make the best use of the available frequency space, use is additionally being made of transmitting a plurality of different signals simultaneously on the same carrier frequency. One example of a communication standard of this kind which uses this principal is the “Universal Mobile Telecommunication System” (UMTS standard) from the “3rd Generation Partnership Project” (3GPP).
In the case of this mobile radio standard the various data to be transmitted are processed in a frequency band with a unique identification code. The data processed with the different identification codes can then be transmitted together on the frequency band. Processing with different identification is called a code spreading method or else “Code Division Multiple Access” (CDMA).
The fact that different data are transmitted simultaneously may result in the amplitude of the total signal fluctuating greatly over time in this frequency band. Whereas the average power of the total signal is relatively constant, for example, individual signal components may have a very high amplitude far above the average. In this case, the probability function for the components arising in the signal which are above the average power is called the “Complementary Cumulative Distribution Function” (CCDF). FIG. 10 shows an exemplary illustration of such a function for a typical WCDMA signal. In this case, it can be seen that the total signal contains components which are up to n dB above the average power. The maximum value which occurs above the average power, which maximum value has a low probability, is called the crest factor.
In the case of the UMTS mobile radio standard, it is possible to use adjacent frequency bands to transmit different, wideband signals simultaneously. Thus, a frequency interval of 5 MHz between the individual carrier frequencies of each frequency band is provided for the UMTS standard. In a base station, which sends signals to different mobile communication appliances, different transmission output stages can be implemented individually for each frequency band. This means essentially parallel processing and a dedicated transmission output stage, including an output power amplifier, for each individual frequency band. Another option is to provide just a single transmission output stage within the base station and to feed a common baseband signal for all the signal sources into said transmission output stage.
FIG. 11 shows a schematic illustration of such a base station for the UMTS/WCDMA mobile radio standard. In this case, the output of the individual WCDMA signal sources, WCDMA-S1 to WCDMA-SM, which provide the signal to be transmitted is connected to a respective shaping filter, S Filter. The digital signal which is output by the sources is interpolated by the shaping filters, which have a root raised co-sign (RRC) shaping response with a “roll-off” of 22%, as prescribed in this mobile radio standard.
The filtered digital signal is then multiplied by a signal from a numerically controlled oscillator NCO and in this way is split over the various frequency bands. The numerically controlled oscillators NCO are chosen such that following the multiplication the individual frequency bands have a respective frequency interval of 5 MHz. The individual frequency bands are then added and are converted into an analog output signal in a digital/analog converter. The output is in turn connected to the transmission output stage (not shown here).
The element in the transmission output stage which is influenced by a high crest factor the most is the individual amplifier stages within the transmission output stage of the base station. To ensure adequate signal quality and, in particular, low error rates, it is expedient for the individual amplifiers to have as linear a response as possible in the region of their input amplitude. This is the only way of ensuring the spectral requirements and the quality of the signal. This means that the operating points of the individual amplifier stages need to be chosen suitably so that the amplifier stages do not reach saturation even at high input amplitudes.
These requirements normally result in the power amplifier being given dimensions which are far too great. This results in additional costs for the individual operators of the base stations and increases the space and power requirement. One alternative option is to alter the input signal upstream of the transmission output stage and in this way to reduce the crest factor. This is possible particularly when the requirements for signal quality and the error rate, the “error vector magnitude” and the “peak code domain error” are low or else are not significantly worsened by the altered input signals. Various options for this can be found, by way of example, in the document by N. Hentati and M. Schrader: “Additive Algorithm for Reduction of the Crestfactor” in 5th International OFDM Workshop, Hamburg, September 2000, pp. 27.1 to .5 or else 0. Väänánen, J. Vankka and K. Halonen: “Effect of Clipping in Wideband CDMA system and Simple Algorithm for peak Windowing” in Proc. World Wireless Congress, San Francisco, May 2002, pp. 614 to 619. FIG. 12 shows a known embodiment of the additive method for reducing the crest factor. In this case, the individual filtered signals are corrected by subtracting additional components from the individual signals in order to reduce the crest factor and hence to avoid distortions within the amplifier stages of the base station.
The processing which has been shown using the additive reduction of the crest factor produces additional spectral components, however, which extend the frequency spectrum and thus result in additional errors in the adjacent channels.